Mitigation measures for High AC Pickup in Pipelines 1

[Thamizhan]

Hi,
We have cross Country Petroleum pipelines that share ROW with HT lines. All our pipelines are cathodically protected (Coating is 3LPE). We have observed Pipeline AC pick up of 17 VAC at one test station where the soil resistivity is 500 Ohm-cm. Temporarily, we have installed 4 nos (20kg) Zn anode(Directly to Pipeline) for grounding.After Zn anodes Installation, the AC pickup at that Test Station drops to 6 VAC. Now we are planning for installation of Polarization cell at that Test Station.
My Question is,
1. Whether the AC current induced in our Pipeline will ground through the Zn anode or travel through the pipeline?
2. How to bring down the AC pickup below 2VAC?

 

[brimmer]

Yes, AC current will go through the zinc anode, at least some, some will stay on the pipeline, will follow path of least resistance, which will be the zinc, until the zinc starts to get consumed and resistivity increases.
You may be able to further reduce the AC by installing additional grounding, but through a device which can decouple DC, such as a Dairyland. In this case you may be able to use other materials such as copper to help ground. You can determine which locations to install these with grounding. Your only other alternative would be to burry wire or ribbon parallel to the pipeline (expensive and impractical usually).
Where you need to be careful is at any above ground appurences, where someone can come in contact with the line, touch step voltage needs to be reduced.
I would ask why you think you need to get as low as 2VAC? This may be impossible depending on the conditions. NACE SP0177 gives guidelines, 15VAC is used as a guideline, however for corrosion it is useful to look at current densities as well.

 

[BigInch]

Pipeline Rules of Thumb

http://books.google.co.uk/books?id=...ine to high tension transmission line&f=false

I hate Windowz 8!!!!